Additive Manufacturing methods and devices have become well-known in various industries for production of parts and products formerly manufactured using subtractive manufacturing techniques, such as traditional machining. Application of such manufacturing methods has not been systematically applied.
By additive manufacturing it means a manufacturing technology as defined in the international standard ASTM 2792-12, which mentions a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining.
The additive manufacturing method may be selected in, but is not limited to, the list consisting of stereolithography, mask stereolithography or mask projection stereolithography, polymer jetting, scanning laser sintering or SLS, scanning laser melting or SLM, fused deposition modeling or FDM.
Additive manufacturing technologies comprise processes which create objects by juxtaposition of volume elements according to a pre-determined arrangement that can be defined in a CAD (Computer Aided Design) file. Such juxtaposition is understood as the result of sequential operations such as building a material layer on top of a previously obtained material layer and/or juxtaposing a material volume element next to a previously obtained volume element.
It is well known by the man skilled in the art that the determination of the voxels geometries and locations is the result of an optimized construction strategy that may take into account the order of the sequential manufacturing operations as related to the capabilities of the chosen additive manufacturing equipment.
The optimized construction strategy typically comprises:
the determination of the geometries and locations of voxels
the determination of the geometries and locations of slices made of a plurality of voxels,
the determination of the orientation of the global arrangement of voxels and/or slices in the referential of the additive manufacturing equipment(s).
the determination of the order according to which the voxels and/or slices are to be manufactured.
A 3D printing device that may be used for the invention is adapted to juxtapose small elements of volume, also referred to as voxel, to build an ophthalmic lens. Furthermore, the 3D printing device may be adapted to lay down successive layers of liquid, powder or sheet material from a series of cross sections. These layers, which correspond to the virtual cross sections from the digital model, are polymerized or joined together or fused to create at least part of the optical equipment.
The primary advantage of this technique is its ability to create almost any shape or geometric feature. Advantageously, using such additive manufacturing methods provides much more freedom during the determining step.
One disadvantage of these techniques is that the object is manufactured using a plurality of element, and then it could be difficult to manage the homogeneity of the final product. This represents a main issue to use this technology to manufacture a transparent ophthalmic lens and more particularly ophthalmic lenses. Indeed ophthalmic lenses should be transparent to respond to the need of a wearer. To be transparent it means that the object manufactured should be homogeneous and without any diffusion.
Within the terms of reference of the invention, an ophthalmic lens is understood to be transparent when the observation of an image through said ophthalmic lens is perceived with no significant loss of contrast, that is, when the formation of an image through said ophthalmic lens is obtained without adversely affecting the quality of the image. This definition of the term “transparent” can be applied, within the terms of reference of the invention, to all objects qualified as such in the description.
Using an additive manufacturing process to manufacture an ophthalmic lens from discrete voxels increases the risk of formation of some defect which could be generated either by a poor homogenization between at least two voxels or by a different level of polymerization inside each voxel due to a bad control of reactivity and mobility of reactive species comprised in said voxel. Then these defects could interact with the light by diffracting it. Diffraction is defined as the light spreading effect that is observed when a light wave is physically limited (J-P. PEREZ—Optique, Fondements et applications 7th edition—DUNOD—October 2004, p. 262). Thus, an ophthalmic lens including such defects transmits an image that is degraded because of this spreading of the light induced by said defects. The microscopic diffraction macroscopically results in diffusion. This macroscopic diffusion or incoherent diffusion results in a diffusing halo and, therefore, in a loss of contrast of the image observed through said structure. This loss of contrast can be likened to a loss of transparency, as defined previously. This macroscopic diffusion effect is unacceptable for the production of an ophthalmic lens. This is all the more so in the case where said ophthalmic lens is an ophthalmic lens, which needs on the one hand to be transparent, according to the meaning defined previously, and, on the other hand, to have no cosmetic defect that can hamper the vision of the wearer of such an ophthalmic lens.
By nature and directly linked to the principle of assembling discrete volume elements, additive manufacturing technologies raise difficulties to manage the bulk homogeneity of the final product. This is particularly striking a problem when one considers manufacturing ophthalmic lens for applications in the visible range. Due to the typical size of voxels considered, typically in the range of 0.1 to 500 micrometers, the objet resulting from an additive manufacturing processes tends to show refractive index variations on a scale which generates scattering (in other terms haze or diffusion) in possible combination with optical distortion. It is therefore a key issue for optical applications to be able to produce parts with sufficient homogeneity in the bulk and sufficient smoothness at the surface not to alter the propagation of light rays and hence minimize scattering phenomena which induce a detrimental loss of contrast.
In addition, the physical constitution of voxels in additive manufacturing technologies classically uses physical means which induce geometry variations for the voxels along the fabrication process. Those physical means can be light induced polymerization and/or thermal management which typically generate dimensional shrinkage at the scale of individual voxels, and also macroscopic stress building at the scale of the object produced by the additive manufacturing process.
As far as optical applications are concerned, these above-described dimensional changes during the manufacturing process, either resulting from dimensional changes at the individual voxel scale or from a collective effect linked to voxel assembling, such as stress build up, directly impact the optical characteristics of the final object and its ability to modify an optical wavefront propagation in a controlled and deterministic fashion across the whole transverse section of a beam being transmitted through a lens. For ophthalmic lenses, such dimensional changes alter the final prescription associated with said ophthalmic lenses and which should be individualized to a particular wearer.
The term “prescription” is to be understood to mean a set of optical characteristics of optical power, astigmatism, prismatic deviation, and, where relevant, of addition, determined by an ophthalmologist or optometrist in order to correct the vision defects of the wearer, for example, by means of a lens positioned in front of the wearer's eye. For example, the prescription for a progressive addition lens (PAL) comprises values of optical power and of astigmatism at the distance-vision point and, where appropriate, an addition value. The prescription data may include data for emmetropic eyes.
It is therefore another key issue for ophthalmic applications to be able to produce an object by additive manufacturing with a sufficient control of the individual and collective voxel geometries so as to deliver a final product whose geometries is in direct relationship with the geometry associated to the initial CAD file
The present invention describes a method to solve this problem by manufacturing a three-dimensional ophthalmic lens with a high management level of the homogeneity during the construction of the ophthalmic lens, through a control of two technical characteristics of the voxel:
the ability to modify their viscosity,
the ability to inter-diffuse together to provide a final homogeneous element.
These two technical characteristics are managed by the choice of component(s) used to manufacture each voxel and by the kind of physical and/or chemical treatment applied to each of them.
In the present invention voxels viscosity levels are controlled along the process sequence so as to manage the inter-diffusion potential of relevant chemical species embedded in the voxels formulation.
The fact that we use viscosity as a key parameter to control the ability of embedded chemical species to inter-diffuse is particularly interesting. The combined control of viscosity and inter-diffusion potential makes a wide range of monomeric and/or oligomeric material formulations accessible for an additive manufacturing production scheme aimed at fabricating transparent homogenous parts